I have had the constant questioning too. When I finally got sick of it, I started using "No" and "It's not your business" alot more. I just heard a radio interview the other day about an author that says that parents shouldn't use the word "No". They should find more creative ways to say no (ie. "We're not doing that right now, but we can do this."). While I do believe there is a place for that, I think kids today have gotten used to that kind of parenting. It's almost to the point where alot of parents forfeit their authority so that kids can be part of the decision-making.
I find using the word "No", and repeating it until the child understands that "no" means "no" is the only way to handle it. Same with "not your business". For school-agers especially, it teaches an important social skill too -- that it's rude to meddle. Kids have to learn that not everything is open for negotiation and that sometimes they're just not allowed to do certain things. Allowing them to think otherwise can set them up for trouble in the future. Try arguing with a cop over the speeding ticket he's giving you, and see where that goes
